Coated metal article and method of making same



United States Patent (3 l ice 3,293,896 COATED METAL ARTHILE AND METHUDOF MAKING SAME Andrew J. Kompanelr, .llr., Lansrlale, Pa, assignor toTelefiex Incorporated, North Wales, Pa, a corporation of Delaware NoDrawing. Filed June 28, 1963, Ser. No. 291,267 5 Claims. (Cl. 72-47) Thesubject matter of the present invention is an improved corrosionresistant shaped metal article, and method for making same.

It is presently conventional practice to form many types of metalarticles to substantially final shape by cold forming operations such ascold extrusion. Typical of articles so manufactured are valves and valvestems, ball studs, switch housings, spark plug shells and the like. Thebillet used for the extrusion or other cold forming operation can beeither cut from ball stock or, as is becoming more the practice, it canbe formed of compacted sintered metal powder.

Many of the steel or other ferrous base articles manufactured by suchtechniques are required to have oxidation and other corrosion resistancein their final form. For example, valves must be highly resistant toerosion or corrosion by combustion gases and switch casings and sparkplug shells must often have at least a certain amount of resistance torusting. The conventional practice is to surface treat the formed metalarticles in order to proide such corrosion resistance. Frequently, thenature of the finished article is such that really good corrosioncoating is not economically feasible and hence bluing, phosphating orother inexpensive treatments are used merely to provide a pleasinginitial appearance and corrosion resistance, which is effective onlyduring the initial period of use. In the case of spark plug shells, forexample, a relatively inexpensive surface treatment is used where theplugs are for automotive use with the result that rusting generallyoccurs after only a few weeks. For marine spark plugs where more durableresistance to salt corrosion and the like is essential, cadmium platingtreatments or the like must be used but at considerable additionalexpense.

It is a principal object of the present invention to provide a methodfor cold forming metal articles wherein during such forming method thearticle is provided with a highly corrosion resistant surface finishwhich can be of pleasing lustrous metallic appearance, if desired.Another and attendant object of the invention is the provision of animproved low cost formed metal article having a corrosion resistantsurface finish.

Briefly, the aforesaid objects are accomplished in accordance with theinvention, by extruding or otherwise cold forming a metal billet havinga surface layer which is provided by coating the billet with a uniformslurry of a finely divided solid particulate material in an aqueoussolution containing major or substantial amounts of phosphate ion, metalion, and ion selected from the group consisting of chromate, molybdateand mixtures thereof. The solid particulate material can, for example,be a metal powder such as powdered aluminum, nickel and silver, aluminumbeing much preferred, or it can be a finely divided dry lubricant suchas molybdenum disulfide, tungsten disulfide, graphite or lead oxide.Frequently it will be desirable to use combinations of different solidparticulate materials, for example, aluminum powder in combination witha powdered dry lubricant.

No claim is herein made to the coating composition per se, such beingcovered by United States patent applications S.N. 291,269, S.N. 291,271,S.N. 291,289 and SN.

3293,89 6 Patented Dec. 27, 1966 291,248, all of said applications beingassigned to the assignee of the present invention.

The phosphate anion concentration should preferably be from 1 to 4 molsper liter though as little as .5 per liter is satisfactory for someapplications of the coatings. The phosphate can be introduced into thesolution in the form of phosphoric acid, in the form of phosphates ofthe metal or metals to be included as the metal cation or, as ispreferred, it can be added in both forms. It will be understood that theterm phosphate is intended to comprehend not only the P0 ion but alsothe H1304 and H PO ions. All three, for example, result from theionization of H PO, and the hydrogen phosphate ions generally will, atleast to some extent, be present in the compositions of this invention.Where phosphoric acid is used as additive in making the compositions, itis much desired to use the ortho acid H PO though the invention alsocomprehends the use of the other forms such as the metal or pyro acidsall ionizing in water to provide phosphate anion. Examples of metalphosphates which can be used to add phosphate ion are the mono, di andtribasic phosphates of magnesium, zinc, aluminum, iron and lithium. Thephosphate used should, of course, be soluble in the solution though thisdoes not mean it must be hi hly water soluble since many of thephosphates not soluble in water are nevertheless soluble in an acidsolution, for example, one containing phos :horic acid or chromic acid.In general, however, the mono and di basic phosphates, which are moresoluble than the tribasic salts, are preferred. The chromate ionconcentration should be from about .3 to 3 mols per liter, at least .5mol per liter being preferred. The chromate can be added as chromicacid, or as a metal chromate or dichromate, for example, the chromate ordichromate of magnesium, zinc, aluminum, calcium, barium, lithium,sodium or silver. If it is desired, to replace the chromate ion in wholeor in part with molybdate ion, molybdic acid or the metal molybdates,for example, zinc molybdate can be used. Chromate ion is, however,preferred.

The metal ion concentration should be from about .2 to 4 mols per literand preferably at least about 1 mol per liter. Where the phosphate and/or chromate or molybdate ion is furnished to the solution by addition ofa metal salt, in accordance with the above, this inherently alsosupplies metal ion to the solution, hence, any of the aforementionedphosphates, chromates, dichromates or molybdates can be used as thesource of metal ion. Additionally, metal ion can be supplied in the formsuch as metal oxide, hydroxide, carbonate, etc. which will dissolve inacid to produce the metal phosphate, chromate, or molybdate andtherefore the metal ion, plus water and/or gas which is evolved. Ofcourse, if the metal ion is added in this form the other solutionadditions must supply the acidity required to accomplish the acid-basereaction. For example, some of the phosphate ion should be added asphosphoric acid, as is preferred, or some of the chromate as chromicacid. The following metal compounds will illustrate those which can beadded to generate the metal ion within the solution in accordance withthe above: Magnesium oxide, magnesium hydroxide, zinc hydroxide,aluminum hydroxide, lithium oxide, lithium carbonate, calcium hydroxide,calcium carbonate. Of course, it will be understood that the insolublerefractory metal oxides, for example alumina, which can be used as thesolid particulate material in the practice of this invention ashereinafter described, cannot be employed as the metal ion additivesince they are difiicult if not impossible to dissolve even in thestrong acid.

In general, the valence +2 and +3 metals are preferred. Magnesium isoutstanding though for some specific uses of the compositions, theferric and aluminum ions which are +3 valence, will serve to advantage.While not as good as magnesium, zinc is also desirable. The valence +1metals are satisfactory though for most applications of the coatings itis generally preferable to use them only in combination with valence +2and/ or valence +3 metals.

The solid particulate material should preferably have a grain size ofless than 325 mesh and should, of course, be insoluble in the solutionand hence form a slurry therewith. It can be included in an amount offrom to 2000 grams per liter though for most uses from about 200 to 800grams per liter will be preferred. Examples of solid particulatematerials are the metal powders, such as powdered aluminum, nickel andsilver, aluminum being outstandingly advantageous, and the drylubricants such as molybdenum disulfide, tungsten disulfide, graphiteand lead oxide.

To attain a coating with exceptionally good lubricity and flexibility,the organic polymers of high inherent lubricity, ideallypolytetrafluoroethylene, can be included in place of or in addition tothe solid particulate material as described and covered by US.applications S.N. 291,- 248 and S.N. 291,358 filed concurrently herewithand assigned to the assignee of the present invention.

In practice, the metal billet is first coated with the slurry of theparticulate material in the phosphate-metalchromate and/or molybdate ionsolution and then dried and preferably cured by heating to a temperatureand for a time sufficient to cause the coating to become waterinsoluble. Then the coated billet is cold formed to desired shape as byextrusion. The coating flows and stretches with the metal withoutcracking or otherwise disintegrating with the result that the shapedarticle has a thin surface film of the cured mixture. Due to the workingof the metal and the coating material during such forming operation, thecoating is somewhat diffused into the metal and takes on properties evensuperior to those prior to such working. The thickness of the coatingapplied to the billet can be from .0005 to .02 inch, the precise optimumthickness depending upon the amount of metal flow in the formingoperation. That is, the initial coating should be thick enough to assurea continuous coating on the finished article. If desired, two or morecoats may be applied. If metal powder, ideally aluminum, is used as thesolid particulate material, the shaped article can be buffed to providea lustrous pleasing metallic appearance. The coating is highly corrosionresistant and by the choice of particulate material, other desirableproperties such as abrasion resistance or lubricity can additionally beimparted. Where the coating includes a dry lubricant, the method has theadditional advantage of increasing tool life. Because of the excellentflexibility of the coating, finishing operations such as thread rollingor the like can be used without disrupting the coating.

If desired the curing operation can be accomplished after rather thanprior to the cold forming operation. This has a disadvantage, however,in that more careful handling of the coated billet is required since theuncured coating has incomparably less strength than the cured coatingand hence can be scratched off in the course of rough conveyingoperations prior to forming and curing.

The following specific examples of the coatings will serve to furtherillustrate.

Example 1 H PO (85% aqueous solution) cc 200 MgO grams 50 do MgCr O .6HO do 160 Aluminum powder (spherical, 5-10 micron) do 800 Water, to cc1000 4 Example 2 CrO grams 100 MP0,; do 122 H PO do 98 Aluminum powder(spherical, 5-10 micron) do 600 Molybdenum disulfide (5-10 micron) do100 Water, to cc 1000 Example 3 H PO aqueous solution) cc 150 ZnCrOgrams 40 1\ IgCI20q.6H2O dO Ni g(H2PO4)2.3H2O dO Teflon dispersion,aqueous (60% solid) cc 400 Aluminum powder (spherical, 5-10 micron)grams 600 Water, to cc 1000 The steel or other metal billet is firstthoroughly cleaned and is then coated as by spraying or dipping with acomposition such as one of the above. Then the coated billet is driedand cured to complete Water insolubility, the precise curing temperatureand time depending upon the composition being used. In general, thecuring temperature will be from 500 to 800 F. and the time of cure from3 to 60 minutes. The lower the temperature, the more time required toaccomplish a full cure. If a lower curing temperature is necessary ordesirable, coating compositions as covered by United States patentapplications S.N. 291,268 and SN. 291,270, now Pat. Nos. 3,248,249 and3,248,250, respectively, assigned to the assignee of the presentinvention, can be used. Briefly, such coatings are the same as thosedescribed above except that they additionally include from about 1 tograms per liter of a soluble metal silicate preferably sodium silicateor a colloidal grain size inorganic gel former such as silica gel oralumina gel, or a combination thereof. With such additives, temperaturesof from 250 to 500 F. can be used to accomplish a full cure.

The coated and cured billet can then be formed to desired final shape byany of the conventional cold forming techniques such as extrusion. Bycold forming is meant forming at room temperature or at least attemperatures, i.e. with a billet temperature, not exceeding about 1000F. though higher temperature may, of course, occur for a very shortperiod during actual metal movement.

While the invention has been described specifically with reference tocertain embodiments thereof, various changes and modifications may bemade all within the full and intended scope of the claims which follow.

I claim:

1. A method for forming a metal article of predetermined shape with acorrosion resistant coating thereon including the steps of coating ametal billet with a mixture of a solid particulate material and anaqueous solution, the solute of said aqueous solution consistingessentially of a combination of inorganic compounds selected from thegroup consisting of phosphoric acid, chromic acid, molybdic acid, andthe metal salts of said acids, the combination of compounds in saidaqueous solution being such as will provide at least .5 mol per literdissolved phosphate, at least .3 mol per liter dissolved material fromthe group consisting of chromate and molybdate, and at least .2 mol perliter dissolved metal, and subsequently cold forming the coated billetto the desired predetermined shape.

2. The method as set forth in claim 1 wherein said solid particulatematerial consists at least in part of aluminum powder.

3. The method as set forth in claim 1 wherein said coating is heat curedat a temperature sufiicient to cause it to become water insoluble priorto said cold forming steps.

4. A method for forming a metal article of predetermined shape with acorrosion resistant coating thereon comprising the steps of coating ametal billet with a slurry of a solid particulate material in an aqueoussolution, the solute of said aqueous solution consisting essentially ofa combination of inorganic compounds selected from the group consistingof phosphoric acid, chromic acid, and the metal salts of said acids, thecombination of compounds in said aqueous solution being such as willprovide from about 1 to 4 mols per liter dissolved phosphate, from about.5 to 3 mols per liter dissolved chromate, and from about 1 to 4 molsper liter dissolved metal, heat curing said coating to cause it tobecome water insoluble and then cold forming the billet to the desiredpredetermined shape.

5. A method for forming a metal article of predetermined shape with acorrosion resistant coating thereon including the steps of cold formingto the desired predetermined shape, a metal billet coated with a driedand heat cured mixture of a solid particulate material and ReferencesCited by the Examiner UNITED STATES PATENTS 1,823,869 9/1931 Baur 29-5282,138,528 11/ 1938 Phillipps 72-47 2,665,231 1/ 1954 Amundsen et al.7246 3,097,978 7/1963 Newell et al. 148-6.16

CHARLES W. LANHAM, Primary Examiner.

L. A. LARSON, Assistant Examiner.

1. A METHOD FOR FORMING A METAL ARTICLE OF PREDETERMINED SHAPE WITH A CORROSION RESISTANT COATING THEREON INCLUDING THE STEPS OF COATING A METAL BILLET WITH A MIXTURE OF A SOLID PARTICULATE MATERIAL AND AN AQUEOUS SOLUTION, THE SOLUTE OF SAID AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF A COMBINATION OF INORGANIC COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF PHOSPHORIC ACID, CHROMIC ACID, MOLYBBDIC ACID, AND THE METAL SALTS OF SAID ACIDS, THE COMBINATION OF COMPOUNDS IN SAID AQUEOUS SOLUTION BEING SUCH AS WILL PROVIDE AT LEAST .5 MOL PER LITER DISSOLVED PHOSPHATE, AT LEAST .3 MOL PER LITER DISSOLVED MATERIAL FROM THE GROUP CONSISTING OF CHROMATE AND MOLYBDATE, AND AT LEAST .2 MOL PER LITER DISSOLVED METAL, AND SUBSEQUENTLY COLD FORMING THE COATED BILLET TO THE DESIRED PREDETERMINED SHAPE. 